A cutting insert having cutting edges on both surfaces thereof is often used because such cutting insert can achieve an increased number of usable cutting edges. Patent Document 1 describes an example of a rotary cutting tool which uses a cutting insert having cutting edges on both surfaces thereof. In this cutting insert, a seating surface is formed so as to be depressed inward with respect to a cutting edge. The seating surface is formed so as to be planar. Patent Document 1 further describes an insert seat (insert pocket) for the mounting of such cutting insert. A base surface (seating surface) of the insert seat has a plane which comes into contact with the cutting insert.